I have a 1972 forcast fiberglass 17ft boat bowrider it has a 1988 mercury 90hp 3 cyl two stoke problem is. when I got the boat the prop was wreck it was a stianless steel 13x19 pitch prop. so a got a new aluminum 13.25x19 wot 4800rpm. I was told the rpm for that motor was 5600.so I change the...